全文获取类型
收费全文 | 276827篇 |
免费 | 3531篇 |
国内免费 | 1003篇 |
专业分类
化学 | 148745篇 |
晶体学 | 3858篇 |
力学 | 10865篇 |
综合类 | 7篇 |
数学 | 33112篇 |
物理学 | 84774篇 |
出版年
2020年 | 2215篇 |
2019年 | 2388篇 |
2018年 | 2938篇 |
2017年 | 2874篇 |
2016年 | 4793篇 |
2015年 | 3360篇 |
2014年 | 4741篇 |
2013年 | 12875篇 |
2012年 | 9688篇 |
2011年 | 11690篇 |
2010年 | 7714篇 |
2009年 | 7420篇 |
2008年 | 10543篇 |
2007年 | 10588篇 |
2006年 | 9748篇 |
2005年 | 9076篇 |
2004年 | 8065篇 |
2003年 | 7079篇 |
2002年 | 6921篇 |
2001年 | 7840篇 |
2000年 | 5921篇 |
1999年 | 4618篇 |
1998年 | 3880篇 |
1997年 | 3903篇 |
1996年 | 3712篇 |
1995年 | 3510篇 |
1994年 | 3365篇 |
1993年 | 3451篇 |
1992年 | 3765篇 |
1991年 | 3618篇 |
1990年 | 3454篇 |
1989年 | 3392篇 |
1988年 | 3433篇 |
1987年 | 3308篇 |
1986年 | 3178篇 |
1985年 | 4414篇 |
1984年 | 4532篇 |
1983年 | 3743篇 |
1982年 | 4089篇 |
1981年 | 3988篇 |
1980年 | 3858篇 |
1979年 | 3878篇 |
1978年 | 4091篇 |
1977年 | 3921篇 |
1976年 | 4113篇 |
1975年 | 3669篇 |
1974年 | 3812篇 |
1973年 | 4067篇 |
1972年 | 2461篇 |
1971年 | 1938篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
R. Cressely V. Hartmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):57-62
In this experimental work we carefully investigate the rheological behaviour and in particular the shear thickening exhibited
by aqueous micellar solutions of CTAB with NaSal as added counterion. We are particularly interested in the evolution of the
critical shear rate (at which shear thickening occurs) versus
C
D
, the surfactant concentration. We show that , at fixed salt concentration C
S
, increases with C
D
following a power law evolution with a positive exponent of + 5.8. On the other hand we show that if the ratio C
D
/ C
S
is fixed, decreases with C
D
with a negative exponent of -2.0. Nevertheless investigations of the zero shear viscosity indicate that in all situations (implying variation of the surfactant concentration C
D
, or the salt concentration C
S
or the temperature) is a decreasing function of the length of the micelles. All these evolutions are compatible with a gelation mechanism which
could possibly be associated with entanglement effects of large interacting flowing structures.
Received: 3 March 1998 / Revised: 16 June 1998 / Accepted: 3 July 1998 相似文献
942.
The optical absorption of GaAs crystals with thicknesses d=0.4−4.4 μm is measured in the exciton-polariton resonance region at a temperature of 1.7 K. As the thickness is reduced,
both a broadening of the exciton line and increased absorption with a negligible Stark shift are observed. The way the absorption
spectra vary with crystal thickness is examined in terms of a competition between two regions for light-exciton interactions
in the crystal: in the field of surface charges and electric-field free.
Fiz. Tverd. Tela (St. Petersburg) 40, 869–871 (May 1998) 相似文献
943.
I. Souza Azevedo E. Baggio Saitovitch M. Hücker B. Büchner H. Micklitz 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):313-315
57Fe M?ssbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K >
T
> 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4.
Received: 23 June 1998 / Accepted: 14 July 1998 相似文献
944.
S.F. Lienin R. Brüschweiler R.R. Ernst 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,131(2):184-190
The viscosity-dependent retarding effect of a polymeric solvent on the rotation of small solute molecules is investigated by13C NMR relaxation measurements. It is found that the relaxation data of 1,3-dibromoadamantane in highly viscous polymeric chlorotrifluoroethene can be explained neither by isotropic nor by realistic anisotropic tumbling in a single environment. The experimental data are rationalized in terms of fast exchange between at least two environments with correlation times differing by up to two orders of magnitude. The study shows that a uniform retardation of molecular tumbling by a polymeric solvent, desirable for shifting the NMR observation window in studies of intramolecular mobility, is not always feasible. 相似文献
945.
Summary A new method of evaluating the Tolman cone angle from X-ray structural data available from the Cambridge Crystallographic Data Base has been developed and a statistical analysis of the cone angles of the phosphines PPh3, PMe2Ph, PMePh2, PMe3, PEt3 and PCy3 (Cy = cyclohexyl) in transition metal complexes has been completed. 相似文献
946.
The motion of a relaxing system can be revealed by its optical properties. the computed emission spectrum of an excited solid-state impurity in a crystal, which relaxes towards the equilibrium position, exhibits a series of peaks revealing the stops at the classical turning points. Detection limits are discussed. 相似文献
947.
A review of otoacoustic emissions 总被引:32,自引:0,他引:32
R Probst B L Lonsbury-Martin G K Martin 《The Journal of the Acoustical Society of America》1991,89(5):2027-2067
Otoacoustic emissions measured in the external ear canal describe responses that the cochlea generates in the form of acoustic energy. For the convenience of discussing their principal features, emitted responses can be classified into several categories according to the type of stimulation used to evoke them. On this basis, four distinct but interrelated classes can be distinguished including spontaneous, transiently evoked, stimulus-frequency, and distortion-product otoacoustic emissions. The present review details the findings that have been described for each emission type according to this classification schema. Additionally, the known features of emitted responses are discussed for both normally hearing and hearing-impaired humans and experimental animals, and with respect to their potential clinical applications. The findings reviewed here clearly indicate that future studies of otoacoustic emissions will significantly increase our understanding of the basic mechanisms of cochlear function while, at the same time, provide a new and important clinical tool. 相似文献
948.
949.
Shelli R. Mellegaard-Waetzig 《Tetrahedron》2006,62(30):7191-7198
Organoselenides catalyze the oxidation of halides by H2O2. Furthermore, these selenides catalyze the transfer of oxidized halogens from N-halosuccinimides to olefins and ketones. Thus, organoselenides catalyze oxidative halogenation reactions including halolactonization, α-halogenation of ketones, and allylic halogenation. The ability of selenium to undergo reversible 2e− oxidation-reduction chemistry facilitates halogenation through selenium-bound halogen intermediates. 相似文献
950.
Xerogels obtained from the acid-catalyzed and ultrasound stimulated hydrolysis of TEOS were submitted to heat treatment at temperatures ranging from 60 to 1100 °C and studied by small-angle X-ray scattering (SAXS). The SAXS intensity as a function of the modulus of the scattering vector q was obtained in the range from q0=0.19 to qm=4.4 nm−1. At 60 °C the xerogels exhibit an apparent surface fractal structure with a fractal dimension DS∼2.5 in a length scale ranging from 1/q1∼1 to 1/qm∼0.22 nm. This structure becomes extremely rough at 120 °C (DS∼3) and at 150 °C, it apparently converts to a mass fractal with a fractal dimension D∼2.4. This may mean an emptying of the pores with preservation of a share of the original mass fractal structure of the wet aged gel, for it had presented a mass fractal dimension D∼2.2. A well characterized porous structure formed by 2.0 nm mean size pores with smooth surface of about 380 m2/g is formed at 300 °C and remains stable until approximately 800 °C. At 900 °C the SAXS intensity vanishes indicating the disappearance of the pores in the probed length scale. The elimination of the nanopores occurs by a mechanism in which the number of pores diminishes keeping constant their mean size. The xerogels exhibit a foaming phenomenon above 900 °C and scatter following Porod's law as does a surface formed by a coarse structure. 相似文献